I want to turn this into something like the 'StudyPass' guides. It needs to be whittled down more. If someone wants to have a go at this, perhaps you could re-write the relevant section and let me know. I will revert it if I think it is too truncated or is inaccurate.
New Zealand is the emergent part of a submerged continent called Zealandia, that has been formed over a period of 500 million years. It was once part of the Gondwana supercontinent, and split off 80-60 million years ago when the Tasman sea formed.
Deposition and uplift
The rocks that form Zealandia have been formed by cycles of deposition, alternating with periods of uplift called orogenies. An orogeny is a period of ‘mountain building’, during which the main processes are uplift (forming land and mountains), volcanism, intrusion, regional metamorphism. Orogenies are often signalled in the geological sequence by widespread unconformity, and the presence of high energy sedimentary rocks such as breccias and conglomerate as well as volcanic, intrusive and metamorphic rocks. Orogenies occur due to the presence of a convergent plate boundary.
The Tuhua depositional period and orogeny.
Before the first orogeny, Zealandia (together with parts of Australia and Antarctica) was entirely underwater. Sediment eroded from Gondwana, to the ‘west’ of this underwater region, was deposited in a huge ‘basin’ off Gondwana’s “east coast”, starting about 500 million years ago. About 400 million years ago, a chain of island volcanoes in this basin signalled the start of convergent tectonic activity, and this escalated until about 370 million years ago when the whole area was uplifted to become land. Remnants of this land are found today in NW Nelson and Fiordland, as well as in eastern Australia and Antarctica. This first period of uplift is called the Tuhua Orogeny.
Rangitata Depositional period and orogeny
The rocks uplifted by the Tuhua Orogeny eroded away, depositing sediment in a basin to the ‘east’, sometimes called the ‘New Zealand Geosyncline’. These sediments were deposited by underwater currents, and form huge thicknesses of alternating sandstone and mudstone collectively known as 'greywacke'. They are hard, grey in colour and have been partly metamorphosed - the schists of Otago and elsewhere are these same sediments, fully metamorphic (deeper in the 'pile').
These sediments in turn was uplifted about 130 million years ago in the Rangitata Orogeny. This was followed by an extensive wearing down of the landscape to a nearly flat surface called a peneplain.
Opening of the Tasman Sea
About 80 million years ago a rift valley formed on ‘east’ of Gondwana. A chunk of eastern Gondwana split off and began to drift away, forming the Tasman Sea and the mini-continent of Zealandia. This continent extended from what is now the Auckland Islands in the south to New Caledonia in the north. The Tasman took some 20 million years to form.
Drowning
Following the opening of the Tasman, Zealandia continued to wear away. It became swampy, forming extensive coal deposits, then gradually sank below the sea. The drowned continent was a shallow and biologically rich sea, full of shells which eventually became limestone. Land life probably clung on in a few scattered islands and sandbanks.
Re-emergence: The Kaikoura Orogeny
About 22 million years ago, the pace of tectonic activity stepped up again. Parts of Zealandia rose out of the sea again, and a chain of volcanic islands formed in what is now Northland. New sedimentary basins formed, which sand and mud eroded from the newly emerged land rapidly began to fill. About 5 million years ago, part of the new plate boundary became a transform fault – the Alpine Fault. Hundreds of kilometres of sideways movement ensued, but a few percent of this movement was vertical and pushed up the Southern Alps.
Ice Ages
About 2 million years ago the Earth entered a period of cold and warm cycles called the Ice Ages. Sea level fell from an initial level some hundred or so metres higher than present to below where it is today, then rose and fell again and again at least eight times. Each time, the new high was a little lower than before. The low periods were called glaciations and were accompanied by the formation of extensive glaciers in Fiordland, the Alps and the central North Island. The last glaciations ended some 12,000 years ago and the sea level rose from 120 metres lower than today back to its current level. These periods between glaciations are called interglacials, and the sea level has started to rise again because human activity has interrupted the natural cycle.
NZ Geological History - summary
I want to turn this into something like the 'StudyPass' guides. It needs to be whittled down more. If someone wants to have a go at this, perhaps you could re-write the relevant section and let me know. I will revert it if I think it is too truncated or is inaccurate.New Zealand is the emergent part of a submerged continent called Zealandia, that has been formed over a period of 500 million years. It was once part of the Gondwana supercontinent, and split off 80-60 million years ago when the Tasman sea formed.
Deposition and uplift
The rocks that form Zealandia have been formed by cycles of deposition, alternating with periods of uplift called orogenies. An orogeny is a period of ‘mountain building’, during which the main processes are uplift (forming land and mountains), volcanism, intrusion, regional metamorphism. Orogenies are often signalled in the geological sequence by widespread unconformity, and the presence of high energy sedimentary rocks such as breccias and conglomerate as well as volcanic, intrusive and metamorphic rocks. Orogenies occur due to the presence of a convergent plate boundary.The Tuhua depositional period and orogeny.
Before the first orogeny, Zealandia (together with parts of Australia and Antarctica) was entirely underwater. Sediment eroded from Gondwana, to the ‘west’ of this underwater region, was deposited in a huge ‘basin’ off Gondwana’s “east coast”, starting about 500 million years ago. About 400 million years ago, a chain of island volcanoes in this basin signalled the start of convergent tectonic activity, and this escalated until about 370 million years ago when the whole area was uplifted to become land. Remnants of this land are found today in NW Nelson and Fiordland, as well as in eastern Australia and Antarctica. This first period of uplift is called the Tuhua Orogeny.Rangitata Depositional period and orogeny
The rocks uplifted by the Tuhua Orogeny eroded away, depositing sediment in a basin to the ‘east’, sometimes called the ‘New Zealand Geosyncline’. These sediments were deposited by underwater currents, and form huge thicknesses of alternating sandstone and mudstone collectively known as 'greywacke'. They are hard, grey in colour and have been partly metamorphosed - the schists of Otago and elsewhere are these same sediments, fully metamorphic (deeper in the 'pile').These sediments in turn was uplifted about 130 million years ago in the Rangitata Orogeny. This was followed by an extensive wearing down of the landscape to a nearly flat surface called a peneplain.
Opening of the Tasman Sea
About 80 million years ago a rift valley formed on ‘east’ of Gondwana. A chunk of eastern Gondwana split off and began to drift away, forming the Tasman Sea and the mini-continent of Zealandia. This continent extended from what is now the Auckland Islands in the south to New Caledonia in the north. The Tasman took some 20 million years to form.Drowning
Following the opening of the Tasman, Zealandia continued to wear away. It became swampy, forming extensive coal deposits, then gradually sank below the sea. The drowned continent was a shallow and biologically rich sea, full of shells which eventually became limestone. Land life probably clung on in a few scattered islands and sandbanks.Re-emergence: The Kaikoura Orogeny
About 22 million years ago, the pace of tectonic activity stepped up again. Parts of Zealandia rose out of the sea again, and a chain of volcanic islands formed in what is now Northland. New sedimentary basins formed, which sand and mud eroded from the newly emerged land rapidly began to fill. About 5 million years ago, part of the new plate boundary became a transform fault – the Alpine Fault. Hundreds of kilometres of sideways movement ensued, but a few percent of this movement was vertical and pushed up the Southern Alps.Ice Ages
About 2 million years ago the Earth entered a period of cold and warm cycles called the Ice Ages. Sea level fell from an initial level some hundred or so metres higher than present to below where it is today, then rose and fell again and again at least eight times. Each time, the new high was a little lower than before. The low periods were called glaciations and were accompanied by the formation of extensive glaciers in Fiordland, the Alps and the central North Island. The last glaciations ended some 12,000 years ago and the sea level rose from 120 metres lower than today back to its current level. These periods between glaciations are called interglacials, and the sea level has started to rise again because human activity has interrupted the natural cycle.